You might want to ask your friend what the mA-hr rating of the original battery is, and then try to match or exceed that number. The mA-hr rating is marked on the battery and is the measure of how much power it can hold.

Some inexpensive batteries may fit, but hold much less power (lower mA-hr rating) so will not run the computer for as many hours as the original does/did.

I purchase replacement batteries for cameras and laptops from highly-rated sellers on eBay and have not had a problem.